Telephone system



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` TELEPHONE SYSTEM 12 Sheets-Sheet 2 Filed May `16, 1940 ANW ATTORNEYS J'une24,1941.` E. OMAX ETAL- 2,246,743

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TELEPHONE, SYSTEM Filed May 16, `1940 12 Sheets-Sheet 7 June 24, 1941.

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ATTORNEYS,

June 24, 1941. C. E LOMAX ET AL 2,246,743

TELEPHONE SYSTEM Clarence/7 Lomaz Otho D Grandsaf ATTORNEYS June 24, 1941.

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/oozQ @a /O /0037 /0/4 al?? c. E.VLoMAx E-r A1. TELEPHONE SYSTEM Filed May 1e, 1940 l2 Sheets-Sheet lO Tandem Exchange Finder Comedor Link Z0 O OO INVENTORS Carence E. Lomax Otho D. G/"cmdsaff ATTORNEYS june 24, 1941. C. E, LOMAX ETAL '2,246,743

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Patented June 24, 1941 Tar erica TELEPHONE SYSTEM Belaware Application May 16, 1946, Serial No. 335,560

68 (Claims.

The present invention relates to telephone systems and more particularly to improvements in remotely controllable line switching apparatus for selectively interconnecting the lines terminating at a switching point.

Line switching equipment of this type is irequently used when only a small number of subscribers lines are served by the equipment and the equipment is necessarily located at a point remote from central exchange facilities. In certain installations of this character it is desirable from the standpoint of initial cost and cost of maintainance to provide an arrangement wherein no current source is required at the switching point for supplying currentV to the component elements of the switching equipment. This is particularly true of the equipment provided at a switching center in a rural area for effecting connections between the small number of lines required to serve the subscribers of the area.

It is an object of the present invention, therefore, to provide in a telephone system of the character indicated, an improved and exceedingly simple arrangement whereby the line switching equipment provided at a switching center may selectively be controlled either directly or through an automatic switch train from an operators position in a distant central exchange, and wherein current for energizing the switching equipment at the switching center is supplied entirely over the control trunk extending to the switching center.

It is another object of the invention to provide a telephone system of the character described, wherein the apparatus provided in the exchange terminating the control trunk for controlling the switching equipment located at the switching center is arranged selectively to impress control voltages designating different lines terminating at the switching center on the different sides oi the control trunk.

It is a further object oi the invention to provide a telephone system of the character described, wherein the apparatus provided in the exchange terminating the control trunk for controlling the switching equipment provided at the switching center is arranged selectively to impress control voltages having diferent polarities upon the diiierent sides of the control trunk in order to elect the desired line switching operations.

It is still another object of the invention to provide a system of the character described, wherein the apparatus provided for selectively impressing the diierent control voltages on the control trunk is selectively controllable through an automatic switch or an automatic switch train from a distant telephone station, such, for example, as a control operators position in a distant central oiiice.

It is a further object of the invention to provide in a system of the character described, an improved arrangement for identifying calling ones of the lines terminating at the switching center when the calls are routed to the control operators position.

It is a still further object of the invention to provide in a system of the character described, an improved line lockout arrangement for guarding the control trunk, when accoupied with a call, against seizure over another of the lines terminating at the switching center.

In the' illustrated embodiments of the invention, the switching equipment provided at the switching centers may selectively be controlled over control trunks directly from an operators position in the exchange terminating the control trunk, or maybe controlled through the automatic switching equipment of a tandem exchange terminating the control trunk from a control operators position in a distant central oiice. In each embodiment of the invention, a group of switching relays is provided at the switching center for performing the required line switching operations. These relays are all energized and controlled over the control trunk extending to the distant exchange. In one embodiment of the invention, only two subscribers lines are terminated at the switching center, provisions being made whereby ringing currents transmitted over these lines are in turn projected over the difierent sides of the control trunk to the distant exchange. In the other embodiment of the invention, three subscribers lines are terminated at the switching center, and provisions are made for rectifying ringing voltages impressed upon two of these lines when calls are initiated thereon, and for impressing the rectified ringing voltages upon the same side of the control trunk with different polarities. In the disclosed arrangements wherein the control trunks are terminated directly at an operators position in a distant exchange, the ringing voltages transmitted over different sides of the control trunks, and of different polarities, are utilized to control apparatus for selectively energizing signal devices individually corresponding to the subscribers lines, thereby to identify the lines upon which the calls are initiated. In the arrangement wherein the control trunks terminate at a tandem exchange, ringing current, regardless of the character thereof, is utilized for the purpose of causing a finder switch to seize the calling control trunk and to route a connection therefrom to the control operators position located in the distant central oice. This apparatus i-s also arranged selectively to impress control voltages upon the two sides oi the calling control trunk for the purpose of controlling the line switching relays at the switching center terminating the calling line to eiect a lockout of all lines terminating at the switching center, other than the calling line.

In the disclosed arrangements, wherein the control trunks are directly terminated at a control operators position, the operator, upon answering a call rou ed over a calling control trunk, automatically eiiects the application of a predetermined control voltage to one side oi the control trunk, whereby the relays located at the distant calling switching center are caused to lock out all lines except the calling line, and to connect the calling line to the control trunk. The equipment provided at the control operators position in association with the calling control trunk may also selectively be controlled by the operator to impress another predetermined control voltage upon 'the control trunk for the purpose of causing the relay equipment in the distant switching center to connect the desired called line to the control trunk, thereby to set up a talking circuit between the calling and called lines. in the other disclosed arrangement, wherein the equipment provided at the switching center is controllable through a tandem exchange from an operators position located in a central office, the operator, upon answering a call routed over one oi' the control trunks, may directively control i pulse-responsive apparatus provided in the tandem exchange through the automatic switching equipment or the tandem exchange, selectively to impress the control voltage designating the called line upon the control trunk eX- tending to the calling or called switching center to operate and establish a connection between the called line and the control trunk.

Further features of the invention relate to the particular arrangement of the circuit elements, whereby the above-outlined and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings in which Figures 1 and 2, when placed end to end in the order named, illustrate a telephone system having certain features of the present invention embodied therein, Mge. l and 8, when placed end to end in the order named, illustrate a second embodiment of the invention, and Figs. l, 3, 4, 5, 5, 7, 9, 10, 11 and l2, when combined in the manner illustrated in Fig. 13, illustrate a more elaborate telephone system wherein the switching centers shown in Figs. 1 and 7 are controllable through the automatic switching apparatus of a tandem exchange from a control operators position in a remote central oiiice.

Referring now more particularly to Figs. l and 2 of the drawings, the system there illustrated comprises a central cnice in which is provided an operators position Si), a distantly located switching center or rural community exchange le, and a trunk l Yinterconnecting the operators posiill tion with the switching center. The switching center terminates three subscribers lines Il, I2 and i3, which may be of the single party or multiparty type, as desired, or as the subscriber requirements and the number of subscribers in the area served by this center dictate. One or more ci the enumerated lines may be o the paystation type ii desired. The lines El, I2 and I3 are illustrated as serving the substations A, B and C,

, respectively, each of which is of the local battery type. Thus, the substation A is illustrated as comprising a transmitter 20a, a receiver Zia, an induction coil 23a, and a local battery 27a, which are arranged to be interconnected through operation oi a hook switch 2?@1l to form a substation of the well-known anti-side-tone type. For the purpose of selectively signaling the substations l associated with the line l i, assuming this line to be of the multiparty type, and of signaling the operator attending the operators position 30, a hand generator 25a of the well-known magneto type is provided at the substation A. This substation also includes a ringer Zta, which is bridged across the conductors of the line Il to be energized by ringing current transmitted over this line. The apparatus provided at the substations B and C is in all respects identical with that provided at the substation A and, accordingly, corresponding reference numerals have been used to identify corresponding parts of the three circuits.

The equipment provided at the switching center l@ includes automatic switching apparatus ier selectively connecting the three subscribers lines il, i2 and i3 to the trunk 3l extending to the operators position -l in the distant central onfce. This apparatus comprises ve relays RIDE, Stili, R526, Rift@ and Rlfl, the last three oi which are line switching relays of the slow-tocperate type and individually corresponding to the lines il and l2, respectively. The relays Rii and Ridi) are effectively polarized by unidirectional conductive devices in the form of rectiers Elib, Hd and M6, respectively. Preferably these rectiers are of the well-known dry disc or copper oxide type. The line switching relays Ril, Elfi@ and RMI! are arranged to be energized over circuits including the trunk conductors Zia and Sib and the windings of a line balancing coil H56. The windings of this coil are of high impedance and, accordingly, serve to block the ilow of alternating current through the windings of the line switching relays.

The trunk Si terminates at the central olce in a trunk circuit 35, which includes a repeating coil and three signal lamp control relays R283, R'Eiii and R226, which are individually energized in response to the initiation of calls on the line i3, i! and l2, respectively. More speciiically, the relay Rll is polarized by a shunting unidirectional conductive device 2&3 to respond to rectified ringing voltage of one polarity transmitted over the trunk conductor 31a, and the relay Ri is polarized by a shunting unidirectional conductive device 253 to respond to rectified ringing current of the opposite polarity transmitted over the trunk conductor Sla. The two unidirectional conductive devices 203 and 2|3 are also preferably in the form of dry disc or copper oxide rectiers. The apparatus embodied in the trunk circuit 35 includes three additional relays R236, R25() and R253, which are provided for selectively impressing voltages of different polar-ities upon the two sides of the trunk 3l in order selectively to energize the line switching relays RiZll, Rl and RMD provided at the distant switching center. In this regard itis pointed out that the exchange battery conventionally provided at the central oilice and having its positive terminal connected to a grounded bus is utilized as one source of current for energizing the line switching relays at the switching center It. An auxiliary direct current source is also provided, this auxiliary source having its negative terminal connected to the grounded bus and having its ungrounded positive terminal connected to the right terminal of the resistor 25'l. The switchboard side of the repeating coil ZEE!!` is multipled to three line jacks 265, 25S and 2M, which individually correspond to the lines ll, l2 and i3. These jacks have associated therewith signal devices in the form of lamps 2'68, ESQ and 2li), which also individually correspond to theilines li, l2 and i3.

For the purpose of handling calls routed to the operators position till, this position is equipped with an operators cord circuit,` which includes an operators telephone set 2H, call and answer plugs 211 and 280, a drop 272 and ringing control i keys 2'l5 and 219. Thesekeysinclude connected to the respective terminals of an alternating ringing current generator 216, and may selectively be operated for the purpose ci impressing voltage of this generator upon the tip and ring contacts oi the plugs Zl'l and 282. Current for energizing the transmitter provided in the operators telephone set 2'l is supplied from the exchange battery through the windings ci a battery feed coil 213.

Briefly to consider the operation of the systein illustrated in Figs. 1 and 2 of the drawings, it is pointed out that calls initiated on the lines Il, l2 and i3 are forwarded to the operator's position 3c to cause the energization of the signal lamps 258, 289 and 21), respectively. A calling subscriber, in initiating a call, is expected to monitor his line in order to determine the idle or busy condition thereof before operating his hand generator to transmit ringing current over the line. Thus, if a call is initiated at the substation A, for example, the receiver 2id is removed from its supporting hook, whereby the hook; switch 22a is caused to operate to connect the transmitter a and the receiver 2id to the line ll. After ascertaining that the` line is idle, the calling subscriber may initiate the call by operating the hand generator 25a. `In response to this operation the switch spring 26a is moved out of engagement with its associated lower contact and into engagement with its associated upper contact to complete a circuit for transmitting ringing current over the line ll. The path traversed by this current extends from the lower terminal of the generator 25a by way of the line conductor Hb, the contacts |34, the winding of Rl lil, the condenser lr6, the conductor lla, and the switch spring 26a to the upper terminal or the generator 25a. When energized over this circuit the relay Rllll operates to complete, at its contacts lll, an obvious path for short-circuiting the condenser ll. At its contacts H3, the relay Rl lll opens a point in the circuit for energizing the relay RlBil. At its contacts H2, the relay Rl Ill completes a circuit for transmitting positive half-cycles of the ringing current over the conductor Sla of the trunk 3l to energize the upper winding of the relay RZl. More specifically, the relay R2 I is, following operation of the relay Rl l0, energized by pulsating direct current over a circuit which extends from ground contacts by way ci the 'rectifier llfl, the contacts ll2 and |34, the conductor Hb, the windings of the generator 25a, the switch spring 26a, the conductor lla, the contacts HM, 24 and Ill, the conductor 3io, the upper winding of Riii, the rectifier 263, and the contacts 256, 2&5 and 255 back to ground. It will be noted that the rectifier 2l3 connected in shunt with the upper winding of the relay R2lil is so poled that it will not pass current of thepolarity transmitted through the rectifier llffl over the conductor tia of the trunk 3 l On the other hand, the rectifier Qili presents a very low impedance to current ci this polarity and, accordingly, the upper winding oi' the relay H2M, which is shunted by this rectifier, is effectively short-circuited. Thus, only the relay B2i@ is substantially energized by the rectined ringing current transmitted over the conductor Stia of the trunk 3l. When thus energized the relay R2 ill iirst closes its X contacts 2 ll to` complete a locking circuit for itself, this circuit extending irorn ground by way of the contacts 255, 245, 2% and 2l l, and the lower winding of Rl) to battery. After this circuit is completed, the relay B2i@ ccrnpletes at its contacts 2l2, an obvious circuit for energizing the signal lamp 268 individually corresponding to the line ll. The illuminated condition of this lamp serves to inform the operator attending the operators position 3l! that a call is present on the trunk 3l and, more particularly, on the line ll, which should be answered.

In order more fully to describe the manner in which the signal lanips 26B, 2tlg and 2li! are selectively energized, it is pointed out that if the hand generator 25e is operated to project ringing current over the line E3 during the initiation of a call at the substation C, the relay Rliiil is energized over a circuit extending from the terminals of the generator 25e andfincluding the conductors lita and ifib, the contacts i122 and the condenser lull. This relay, in operating, opens, at its contacts lii, a point in the operating circuit for the relay and completes, at its contacts lill, an obvious alternative path for shortcircuiting the condenser ll. At its contacts W2, the relay Riilli completes a circuit including the rectifier ltli for transmitting the negative halicyclesoi the ringing current over the trunk line conductor 3io, to energize the upper winding of the signal lamp control relay RZBB. More specifically, the circuit for energizing the relay Rilll extends from ground by way of the contacts 255, 245 and 236, the upper winding of 12,2%, the rectifier 2id, the conductor Sla, the contacts lul, l32 and M5, the conductor leb, the windings of the generator 25e, the switch spring 2te, the conductor i3d, the contacts i22 and m2, and the rectifier lut back to ground. The rectifier H35 is so poled that the current traversing this circuit will not pass through the rectiiier Zii but is readily passed by the rectiiier 2li?. Accordingly, the relay Rdll energized by the pulsating direct current traversing its upper winding. The action of the rectiiier Zit to pass the rectiiied ringing current transmitted over the line i3 effectively short-circuits the upper winding oi the relay R2 it so that this relay is not substantially energized. The relay R290, in operating, locks to ground through the contacts 25d, 25H, 23B, 245 and At its contacts 2ll2, the relay R200 completes an obvious circuit for energizing the signal lamp Elli, which corresponds to the line i3.

irom the above explanation it will be apparent that the two relays REG@ and REM are operative in response to oppositely poied half-wave ringing currents transmitted over the upper line conductor 3m of the trunk 3|, selectively to control the energization of the signal lamps 258 and 270, which individually correspond to the lines l and 3. The signal lamp control relay R220, which cor-responds to the line i2 and serves to control the signal lamp 259, on the other hand, is energized by alternating ringing current transmitted over the lower line conductor Sib of the trunk 3|. Thus, if the hand generator 25h provided at the substation B is operated during the initiation of a call at this substation, ringing current is transmitted through the upper winding of the relay R220 over a circuit which extends from ground by way of the contacts 253, 2152, 235 and 222, the upper winding of R220, the conductor 3|b, the condenser |52, the contacts |3| and |2|, the conductor |2b, the windings of the generator 25o, the switch spring 22o, the conductor lila and the contacts lill back to ground. When energized over this circuit, the relay R222 operates and locks to ground over a path including the contacts 22d, 235, 2614 and 253. At its contacts 223, the relay R225 completes a circuit including the contacts 235, 2li@ and 253 for energizing the signal lamp 25S which corresponds to the line l2.

In order to answer a call routed to the operators position over the trunk 3| from one of the lines terminating at the switching center I0, the operator attending this position may insert one of the plugs 211 and 280 of her cord circuit in the jack which corresponds to the calling subscribers line. Thus, if the signal lamp 268 is energized, indicating that a call is present on the line the operator may insert the plug 211 in the jack 265. In response to this operation, negative Abattery potential is impressed upon the sleeve contact of the jack 255 through the resistor 278 to energize the relay R230. This relay, in operating, opens, at its contacts 235, a point in the above-traced locking circuit for the relay R220, and a point in the circuit for energizing the signal lamp 269. At its contacts 235, the relay R230 opens the locking circuit for the relay R2 0, and a point in the locking circuit for the relay R200. The relay R2i0 is thus deenergized and restores to open, at its contacts 2|2, the circuit for energizing the signal lamp 263. With the relay R230 operated, the locking circuits for all three of the signal control relays R200, R2|0 and R222, are held open. At its contacts 23|, the relay R230 bridges the windings 26| and 232 of the repeating coil 260 across the conductors of the trunk Iii in series with the condenser 231. At its contacts 232, the relay R230 completes a path including the resistor 255, the contacts 232 and the winding 26|, for impressing negative battery potential upon the trunk conductor 3|a. When this path is completed, the slow-to-operate line switching relay R|,30, individual to the line is energized and operates. More specically, the circuit for energizing the relay Rl 30 extends from ground through the rectifier ||ll and by way of the contacts H3, the winding of RIS, the upper winding of the balancing coil |50, the conductor (Ma, the winding 26|, the contacts 23| and 232, and the resistor 255 to the negative terminal of the exchange battery. The rectier H4 included in this circuit is so poled that its resistance to current flow is substantially negligible. It will be noted that the relay R|20 and the rectiier |25 are connected in parallel with the relay R|30 and the rectier I4. The rectier is, however, so poled. that' it blocks the ow of current through the relay R|20 when negative battery potential is impressed upon the trunk conductor 3|a. Thus, only the relay R|30 is energized. In operating, the relay R|30 opens, at its contacts I3|, a point in the above-traced path for transmitting ringing current from the line |'2 over the lower trunk conductor 3|b to the relay R220 embodied in the trunk circuit 35. At its contacts |32, the relay R|30 opens a point in the abovetraced path for transmitting rectified ringing current from the line I3 over the trunk conductor 3|a to the relays R200 and R2| 0 embodied in the trunk circuit apparatus 35. At its contacts |34, the relay R similarly opens a point in the above-traced path for transmitting rectiiied ringing current from the line over the upper trunk conductor 3|a to the relays R200 and R2l0. Thus, the circuits, individual to the lines I2 and I3, for transmitting signaling currents over the trunk 3| to energize the relays R200, R2|0 and R220, are interrupted.

At its contacts |35, the relay R|30 opens a point in one of the circuits for energizing the relay Rl2i. At its contacts |31, the relay R|30 prepares another circuit for energizing the line switching relay R|20. At its contacts |33 and |35, the relay RIS@ connects the conductors of the calling line through the condensers |5| and 52 to the conductors of the trunk 3| and, in so doing, establishes a communication connection between the calling substation A, for example, and the operators telephone set 27|. After this connection is established, the operator attending the operators position 30 may converse with the calling subscriber to ascertain the substation desired. If the call initiated at the substation A is of the revertive type, the operator will ascertain this fact by monitoring the connection for a short time interval.

Ii the calling subscriber desires the assistance of the operator in setting up a reverting call connection, the operator may either instruct the calling subscriber as to the code assigned to the desired substation and then retire from the connection by withdrawing the plug 271 from the jack 235, or she may signal the called substation directly by operating the ringing key 275 in accordance with the code assigned to this substation. In the latter case, ringing current is projected over the trunk 3| and the calling line each time the ringing key 275 is operated to its oir-normal position. The local circuit traversed by this current may be traced as extending from the left terminal of the ringing current generator 276 by way of the switch springs 275a, the tip contact of the plug 271, the tip contact of the jack 265, the repeating coil windings 26d and 253, the ring contact of the jack 265, the ring contact of the plug 277, and the contacts 275b to the opposite terminal of the ringing current generator 216. The alternating character of the current traversing this circuit causes, by induction, current to traverse a circuit extending from one terminal of the ringer provided at the called substation by way of the line conductor la, the contacts |33, the condenser |5i, the trunk conductor Sia, the winding 26|, the contacts 23|, the condenser 237. the winding 262, the trunk conductor 3|b, the condenser |52, the contacts |35, and the line conductor ||b to the opposite terminal of the ringer provided at the called substation. Thus, the ringer provided at the called substation is energized each time the ringing key 275 isoperated by the operator handling the call. The codedv character of the signals produced through operation of the ringers provided at the substations associated with the line Il serves to inform the called subscriber that his substation is being signaled and to inform the subscribers at the other substations that they are not to answer the call. In the event the signaling operation is performed at the calling substation A by the calling subscriber, the operator, upon ascertaining this fact, may release the operated switching equipment after the ringing operation is ended and the call is answered, by withdrawing the plug 277 from the jack 265. In response to this operation the relay R230 is deenergized and restores to open the above-traced circuit for energizing the relay Rltil provided at the switching center It. The relay RISC), in releasing, disconnects the line Il from the trunk 3l. Thus, the apparatus provided at the switching center Hi and also that embodied in the trunk circuit 35 is fully released. If the called party is served by a substation line terminating at the central oidce, the operator attending the position 3c may extend the connection to the desired line in the manner conventionally followed in manual office practice.

If the call originating at the substation A is intended for a substation such, for example, as the substation B associated with the line i2, the operator handling the call at the operators position Sil may obtain a connection with the desired line by inserting the other plug 235 of her cord o circuit in the jack Z55-3 corresponding to the line l2. In response to thisoperation, negative bat tery potential is impressed upon the sleeve contact of the jack 256 through the resistor 2M to energize the relay RMU. operating, opens, at its contacts 2M and 2655, additional points in the above-traced holding circuits tor the relays Ritt, Rill@ and Ri). At its contacts liti, the relay R265] completes an obvious shunt across the operated contacts 238 of the relay Rll. At its contacts M3, the relay Rlllt completes a path including the resistor 258 and the repeating coil winding ttt for impressing negative battery potential upon the lower trunk conductor 3 ib. W hen this path is completed, the relay Ridil is energized and operates. More spe cically, the circuit for energizing the relay Rill@ extends from the grounded terminal of the eXn change battery provided in the central oiiice by way of the rectiiier hlt, the winding of Ritt, the lower winding of the balancing coil i563, the conductor Zilla, the winding 2d?, the contacts and the resistor 258 to the negative terminal of the exchange battery. It will be noted that with the relay Riti@ operated, a parallel branch of the circuit just traced extends through the contacts 37, the winding of Riii, the contacts M33 and the rectiiier itt. The rectier 85 is, however, so poled that it will not pass current through the winding of the relay Ritt when negative battery potential is impressed upon the upper winding terminal of the relay Rii. Aocordinrly, only the relay Riil is energized in resp-onse to operation of the relay Ritt e1nbodied in the trunk; circuit The relay Bildt, in` opera et its contacts iti, itil and M5, additional points in the above-traced sig naling circuits. At its contacts M2 and M3, the relay Ritt connects the conductors of the desired or called line i2 through the condensers |57 and lili to the conductors of the trunk 3l. Thus, a th.reeway connection involving the lines il and i2 and the trunk Si! is established.

After this connection is set up, the operator may monitor the called line for a short time in- The relay RZMS, in c CTL terval in order to determine the idle or busy condition thereof. Assuming the called line i2 is idle, the" operator may signal the called substation B by operating the ringing key 279 in accordance with the ringing code assigned to the substation B. Two methods of signaling the called substation may be followed. Thus, the operator may instruct the calling subscriber to hang up, and then operate the ringing key 279 to transmit ringing current over both the calling and called lines. In this case the path traversed by the ringing current, each time the ringing key 279 is moved oil normal, extends from the left terminal of the ringing current generator 276 by way of the contact springs 279er, the tip Contact of the plug 23d, the tip contact of the jack 256, the repeating coil windings 254i and E53, the ringing contact of the jack ttt, the ringing contact of the plug 23d, and the contact springs 279i) to the opposite terminal of the ringing current generator. The alternating character of this current causes induced voltages to be developed in the repeating coil windings 25| and 252. Thus, `ringing current is caused to traverse two branch circuits which include, in parallel, the ringers bridged across the conductors of the called line it and the ringers bridged across the calling line li. Thus, the ringer 2th provided at the called substation B is energized over a circuit which extends by way of the trunk conductor da, the condenser ll, the contacts M2, the conductor ma, the windings of the ringer Mb, the conductor lib, the contacts H43, the condenser itt, and 4the trunk conductor 3th. After operating the ringing key 279 in accordance with the code assigned to the called substation B, the operator attending the position 3i] may monitor the connection to ascertain whether or not the call is answered at the called substation. In this regard it will be apparent that a three-way communication connection is established between the calling substation A, the called substation B, and the operators telephone set 277, when the call is answered at the called substation B and the calling subscriber again removes the receiver 2id from the supporting hook of the hook switch 22a.

An alternative method of signaling the called subscriber, which may be followed by the operator handling the call, is that of removing the answer plug 277 from the jack before operating the ringing key tilt in accordance with the code designating the desired substation. In such case the relay Ric is deenergized to open the operating circuit for the relay Ritt when the plug 277 is withdrawn from the jack 2te. Thus, the two relays Rte and Ritt are deenergized and restored. The relay Ritt, in releasing, disconnects the conductors of the line li from the trunk 2li. Accordingly, when the key i279 is operated to transmit ringing current over the trunk Sti, this current is only projected over the called subscribers line l2 to energize the ringers provided at the substation served by this line. After the ringing operation is completed and the call is answered, the operator handling the call may again insert the plug 277 in the jack 255 to cause the sequential reoperation of the relays` R23!) and Ritt, whereby a three-way connection between the subscribers lines il and i2, Vand the trunk tl, is again completed. Thus, a threeway communication circuit is established between the calling and called substations and the operators telephone set i711.

If the call routed to the operators position Sil from the calling substation A is intended for a substation, such, for example, as the substation C associated with the line I3, the operator may obtain a connection with the desired line I3 by inserting the plug 280 in the jack 251. In response to this operation, negative battery potential is impressed through the resistor 23| upon the sleeve contact of the jack 251 to energize the relay R250. This relay, in operating, opens,

at its contacts 253, 254 and 255, points in the above-traced locking circuits for the signal lamp control relays R230, R2I0 and R220. At its contacts 25|, the relay R250 completes an obvious shunt across the operated contacts 23| of the relay R230. At its contacts 252, the relay R250 completes a path including the resistor 251, the contacts 234 and 232, and the winding 232 for impressing the positive potential of the auxiliary current source provided in the central office, upon the lower trunk conductor 3Ib. In response to this operation, the relay RI20, individual to the line I3, is energized and operates. More specifically, the circuit for energizing the relay RI20 extends from the grounded negative terminal of the auxiliary current source by way of the rectier |05, the contacts |03, the winding of RI20, the contacts |31, the lower winding of the balancing coil |50, the conductor 31h, the winding 262, the contacts 242, 234 and 252, and the resistor 251 to the positive terminal of the auxiliary current source. It will be noted that a branch of this circuit extends through the relay Ri@ and the rectifier |35. The rectifier |35 is, however, so poled that it will not pass current through the relay R140 when the positive terminal of the auxiliary current source is connected to the trunk conductor 3Ib in the manner just described. Accordingly, only the relay RI20 is energized. This relay, in operating, opens, at its contacts |2I, |22 and |24, additional points in the previously traced signaling circuits. At its contacts |23 and |25, the relay RI20 connects the conductors of the called line I3, through the condensers I5| and |52, to the conductors of the trunk 3|. Thus, a three-way connection between the subscribersl lines |I and I3, and the trunk 3|, is established. After this connection is set up and when the operator actuates the ringing key 219 in accordance with the code assigned to the called substation C to project ringing current over the trunk 3 I, the ringer 24e provided at the called substation is energized. The circuit for energizing this ringer includes the conductor 3Ia, the condenser I5I, the contacts |25, the conductor |311, the windings of the ringer 23o, the conductor I3a, the contacts |23, the condenser |52, and the trunk conductor 3Ib. In this regard it will be recalled from the above explanation that, if the second described method of signaling the called substation is followed, the answer plug 211 is withdrawn from the jack 235 before the ringing operation is started. In such case1 the relays R230 and RI30 are deenergized and restore. These two relays, in releasing, open two points in the initially completed circuit for energizing the relay RI20 and complete an alternative circuit for energizing this relay. This alternative circuit extends from the grounded negative terminal of the auxiliary current source by way of the rectifier |05, the contacts |03, the winding of RI20, the contacts |33, the upper Winding of the balancing coil |50, the trunk conductor Sla., the repeating coil winding 25|, the contacts 25|, 233 and 252, and the resistor 251 to the positive terminal of the auxiliary current source. Thus, the control circuit for the relay R|20 is shifted from the lower trunk conductor 3Ib to the upper trunk conductor 35a. in response to the release of the relays R230 and RI33, but the relay RI20 is held energized to maintain the 4connection between the called line I3 and the trunk 3|. It will be noted that a branch of the auxiliary circuit for energizing the relay RI20 extends through the winding of the relay RI30, the contacts I I3 and the rectier I I4. The rectier H3 is, however, so poled that it will not pass current through the Winding of the relay RI 30 when the positive terminal of the auxiliary current source provided in the central office is connected to the upper trunk conductor' 3Ia. Accordingly, only the relay RI20 is energized over this auxiliary circuit. After the ringing operation is ended and the call is answered at the called substation, the operator may reinsert the plug 211 in the jack 255 to cause the reoperation of the two relays R230 and RI30. In this regard it will be noted that the relay R230, in operating, first opens, at its contacts 233, the path over which the positive potential of the auxiliary current source is impressed upon the upper line conductor 31a, before it completes, at its contacts 232, the previously traced path for impressing the negative potential of the exchange battery upon the upper trunk conductor Sla. The relays RI30 and R233, in reoperating, open the auxiliary circuit for energizing the relay RI20 and recomplete the initially traced operating circuit for this relay. The relay RI30, in operating, again connects the calling line II to the trunk 3| to establish a three-way connection between this line, the called line I3 and the trunk 3|.

The release of either of the two connections set up in the manner explained above is entirely under the control o-f the operator attending the operators position 30. In order to signal the operator that the connection is no longer desired, the calling subscriber may operate the hand generator 25a, provided at the calling substation A, to transmit ringing current by way of the line II, the trunk 3|, and the repeater 230, thereby to energize the drop 212 embodied in the operators cord circuit. Alternatively, the ring-off operation may be performed at the called substation. When the drop 212 is thus energized, it operates in a conventional manner to inform the operator that the connection is no longer desired. To release the connection the operator withdraws the plug 211 from the jack 235 and the plug 230 from the one of the two jacks 255 and 261 in which it is inserted. When the plug 211 is withdrawn from the jack 255, the relays R230 and RI30 are sequentially deenergized and restore. Similarly, if the plug 280 is withdrawn from the jack 256, the relays R240 and RIliIl are sequentially released in a manner clearly apparent from the above explanation. Alternatively, if the plug 230 is withdrawn from the jack 251, the relays R250 and RI20 are sequentially deenergized and released. Thus, when the plugs 211 and 283 are respectively withdrawn from the jacks corresponding to the calling and called subscribers lines, all of the relays embodied in the trunk 4circuit 35 and all of the relays provided at the switching center I0 are caused to release. Thus, the equipment is conditioned to handle another call.

From the above explanation it will be apparent that, if the call routed over the trunk 3| to the operators position 33 originates at the substation B, for example, associated with the line I2, the operator will insert the plug 211 in the jack 266, in order to answer the call. In response to this operation, the relay R240 is energized over a circuit including the sleeve contact of the jack 206 and operates to open the locking circuits for the signal lamp control relays R200, R2I0 and R220. At its contacts 243, the relay R240 completes the previously traced path for impressing negative battery potential upon the trunk con` ductor Sib, thereby to energize the relay RI40. The relay R|40, in operating, opens the circuits for transmitting ringing current over the trunk 3|, and, at its conta-cts |42 and |43, connects the calling line to the trunk 3|. tion is established the operator may route the call to a desired substation associated with either of the `two lines I I and I3, by inserting the plug 2&0 in the jack 2%, or the jack 261. In this regard it will be understood from the above description that, ii the plug 230 is inserted in the jack 255, the relays R230 and R 30 are sequentially energized. Further, the relay RISE) functions to conneet the line II to the trunk 3|. On the other hand, if the plug 230 is inserted in the jack 261, the relays Ritil and RI20 are sequentially energized and the latter relay, in operating, connects the line I3 to the trunk 3| to establish the desired three-way connection. By way of completing the explanation, it is pointed out that, if the substation A, for example, associated with the line` Il is desired by the calling party, ringing current may be transmitted by Way of the line I I to energize `the ringer 2da over a circuit which includes the trunk conductor Sia, the condenser |56, the contacts E33, the line conductor IIa, the windings of the ringer 2da, the line conductor IIb, the contacts i3d, the condenser, E52, and the trunk conductor .'iIb. The manner in which the desired substation is signaled, the talking circuit between the calling and called substations is established, and the operated switching equipment is released, will be clearly apparent from the preceding explanation.

` .lxf the call routed to the operators position 30 originates at a substation such, for example as the substation C associated with the line I3, the signal device 2lb associated with the jack 201 is energized and, accordingly, the operator will insert the plug 211 in the jack 261 to answer the call. In response to this operation, the relays R250 and RIZ@ are sequentially energized in the manner previously explained. The relay RIZEI, in operating, connects the calling line I3 to the trunk: ."il to establish a communication connection between the calling substation and the oper ators telephone set 2l i. After this connection is established the call may be extended to a substation associated with either of the two lines I I and I2, ina manner clearly apparent from the above explanation.

Calls initiated on the central oflice subscribers lines and routed to the operators position 30 may be extended to the substations served by thelines l I, I2 and i3 under the control of the operator attending this position, in a manner which will readily be understood from the above description. Thus, to extend a call to the line II, the operator insert either of the two plugs 2li and 28@ in the jack 2&5, to cause the sequential operation of the relays R230 and RI30. The relay Rid, in operating, establishes a connection between the line Il and the operators cord circuit, so that the operator may signal the desired substation by operating one of the two ringing After this connec-ikeys 215 and 219. Similarly, either of the two plugs 211 and 280 may be inserted in the jack 266 to cause the operation of the relays R240 and RMB, whereby a connection is established between the subscribers line I2 and the operators cord circuit. Finally, if one of the plugs 211 and 280 is inserted in the jack 261, the relays R250 and R|20 are caused to operate to establish a connection between the line I3 and the operators cord circuit. From the foregoing description it will also be apparent that the line switching relays R220, Rlll and Riti), provided at the switching center Iii, are selectively controlled in accordance with the polarities of thepotentials impressed upon the two sides of the trunk 3 I. Thus, to energize the relay Ri20 individual to the line i3, positive potential of the auxiliary current source may be impressed upon either side of this trunk. To energize the relay Ritt individual to the line i i, the negative potential of the exchange battery is impressed upon the upper trunk conductor Sla. Finally, the relay Ridi? individual to the line l2 is energized in response to the application of the negative potential of the exchange battery to the lower trunk conductor 3Ib. It will also be observed that, with any one of the relays Riti?, Rieti and Riti] operated, the corresponding line is connected to the trunk 3| and the other two lines are prevented from obtaining access to this trunk.

The system formed by combining Figs. 7 and 8 of the drawings is, in general, similar to that illustrated in Figs. l and 2 of the drawings and described above, and the same general method is used in handling the calls originating on the lines I4 and l5 serving the subscribers of the outlying cornrnunity. In the system shown in Figs. l and 8, however, only two subscribers lines are provided to serve the subscribers of the small community area. These lines may be of the single party, multiparty or paystation type and are illustrated as serving the local battery substations D and E, respectively. Since the two substations D and E are identical with the substations A, B and C, illustrated in Fig, 1 of the drawings, corresponding reference numerals have been used to identify corresponding parts of the various substation circuits. The two subscribers lines I and I5 terminate at a remotely con trolled exchange or switching center 10, which is connected by means of a trunk with a distantly located central omce. The switching cen ter 10 comprises automatic switching apparatus in the form of a pair of slow-to-operate relays R and R1 i 0 for selectively connecting the two lines Ill, and I5 to the trunk 55. More specically, these relays individually correspond to the lines l5 and I 4, respectively, and are arranged to be energized over circuits including the trunk conductors 55o and 55h, respectively, and the windings of a line balancing coil from the current source provided in the central office. The windings of the coil 120 are of high im pedance and accordingly function to block the flow of alternating current through the windings of the line switching relays R100 and R'II. The trunk terminates in the central oflice in a trunk circuit @0, which is associated with the op erators position 5. This ltrunk circuit includes a repeating coil 000, a pair of signal lamp control relays ROI0 and R020, and a pair of line switching control relays R330 and R046, the latter relays being arranged respectively to control the energization of the relays R'IIEI and R100 provided at the switching center 10. The switchboard side of the repeating coil 800 is multipled to the line contacts of the two jacks 85I and 852 which individually correspond to the lines I and I4 and are located at the operators position B5. These jacks have associated therewith signal devices in the form of signal lamps 853 and 554 which also individually correspond to the subscribers lines I5 and I4. The operators position 85 is further equipped with an operators cord circuit which may be identical in arrangement with that illustrated in Fig, 2 of the drawings as being provided at the operators position 3l).

Briefly to consider the operation of the system illustrated in Figs. 7 and 8 of the drawings, if a call is initiated at the substation D, for example, the receiver 2Id is removed from its supporting hook 22d, thereby operatively to asso-- ciate the substation D with the line I4. After a connection is thus established to the line I4, the calling party may monitor this line to determine the idle or busy condition thereof. Assuming that the line is idle, the calling party may actuate the hand generator 25d for the purpose of signaling the operator attending the operators I position 85 in the distant central ofiice. In response to operation of the generator 25d, the switch spring 25d is disengaged from its associated lower contact and operated into engagement with its associated upper contact to connect the output windings of the generator across the line I4. After this switching operation occurs, the ringing current developed through operation of the generator 25d serves to energize the upper winding of the relay RIII over a circuit extending from ground by way of the contacts "H3, the conductor Illa, the switch spring 26d, the windings of the generator 25d, the line conductor Mb, the contacts '154, the condenser 165, the trunk conductor 55h, the upper winding of RSI@ and the contacts 8i I, 834 and 344 back to ground. W'hen thus energized, the relay RBIII operates and locks to ground over a path including the contacts SI2, 854 and 844. At its contacts SI2, the relay RSIO completes an obvious circuit for energizing the signal lamp 854. The illuminated condition of this lamp serves to inform the operator attending the operators position S5 that a call is present on the line I4 which should be answered.

In order to answer the call, the operator may insert the answer plug of her cord circuit in the jack 552. In response to this operation, battery potential is impressed upon the sleeve contact of the jack 852 to energize the relay R535. In 0perating, the relay R830, at its contacts 855, opens a point in the operating and locking circuits for the relay R525. At its contacts 334, the relay R235 opens the above-traced circuits for energizing the relay RBII'I and the signal lamp 854, When thus deenergized the relay R8I0 restores. At its contacts 332, the relay R835 completes a path for bridging the windings Silea and 85th of the repeating coil 855 across the conductors of the trunk 55 in series with the condenser SGI. At its contacts 533, the relay R535 completes a circuit for energizing the slow-to-operate relay R'IIO provided at the switching center III. This circuit extends from ground by way of the winding of R'Ht, the winding E22 of the balancing coil "E20, the conductor 55D, the repeating coil Winding 8051), the contacts 333 and the resistor 835 to battery. The relay R'Il, in operating,

, opens its contacts H3 to disconnect the line conductor Illa from ground, and thus open a point in f the above-traced circuit traversed by the ringing current. At its contacts 'III, the relay RHI! opens a point in the circuit for transmitting ringing current over the trunk when a call is initiated at one of the substations associated with the line I5. Thus, the line I5 is effectively prevented from obtaining access to the trunk 55. At its contacts H2 and H4, the relay R'IID connects the conductors of the line I4 to the conductcrs of the trunk 55 through the condensers ilrand 555. Thus, a talking circuit is established between the calling substation D and the telephone set embodied in the operators cord circuit in use at the operators position 85. After this communication circuit is established, the operator may challenge the connection to determine whether or not the call is intended for another substation associated with the line I4. In such case, the operator may release the trunk 55 by withdrawing the answer plug of her cord circuit from the jack 852, thereby to cause the sequential deenergization of the relays R830 and R'III), in an obvious manner. After these relays restore, calls may be routed to the operators position from the substaticns associated with the line I5, in the manner described hereinafter, even though the line I4 is occupied with a reverting call. If the assistance of the operator is required by the calling subscriber in order to set up the desired reverting call connection, the operator may instruct the calling party as to the code assigned to the called substation, whereby the calling subscriber may signal the called substation directly. Alternatively, the operator may instruct the calling subscriber to hang up and signal the called substation from her position by operating the ringing key embodied in her cord circuit, in accordance with the code designating the desired substation. In the latter case, ringing current is transmitted over the tip and ring contacts of the jack 852, through the repeating coil 85) and over the trunk 55 and the subscribers line I4, to energize, in parallel, the ringers respectively provided at the substations associated with the subscribers line. After the ringing operation is completed, the operator may monitor the connection to ascertain that the desired talking circuit is established, and then release the trunk circuit and the switching equipment at the switching center l0, by withdrawing the answer plug of her cord circuit from the jack 852. In response to this operation, the relays R835 and RIID are sequentially released in the manner explained above. Thus, the trunk 55 is rendered accessible to the other subscribers line I5 terminating at the switching center TI).

Assuming that the above-described call originating at the substation D is intended for a subscribers line terminating at the central oiiice, the operator may extend the connection to the desired line in the manner conventionally followed in manual oice practice. If the call originating at the substation D is intended for a substation, such, for example, as the substation E, associated with the line I5, the operator may establish a connection with this line by inserting the calling plug of her cord circuit in the jack SEI. In response to this operation, negative battery potential is impressed upon the sleeve contact of the jack S5I to energize the relay R545. The relay R840, in operating, opens, at its contacts 84I, a further point in the circuits for energizing the relay R820 and the Signal lamp 853. At its contacts 842, the relay R840 

